The objective of this research is to understand the neuronal basis of behaviors: how they are produced, initiated, maintained, and coordinated. To do this, we use the nervous system of the medicinal leech, because it is relatively simple and highly accessible while the animal performs many different behaviors: bending, shortening, crawling, feeding, and swimming. To be sure that we understand how these circuits work, we construct computational models that incorporate the significant features of real neurons. These models therefore, serve as a feasibility test, to make sure that we are not missing major parts of the circuit. To continue this work, we will perform the following sets of experiments: 1. We will find the neuronal basis for how one response (whole-body shortening) dominates another behavior (swimming). 2. We will determine how the crawling behavior is produced by neuronal interconnections, and how they use neurons that produce other behaviors (esp. swimming and shortening) to produce crawling. 3. We will investigate how the same interneurons can be used to produce different behaviors, and how this multiplexing of neurons affects coordination and behavioral choice. Each of the organizational features to be studies is found in all complex animals, including our own. It is important that we understand completely how these mechanisms work in any system, so that we will have a rational basis for therapeutic aids when our own brains malfunction or are injured. Using the leech, we should be able to find every neuron responsible for each of these functions and to determine how they work as a unit to produce normal behavior.